Auto merge of #2577 - alexcrichton:doc-lib-flags, r=brson
authorbors <bors@rust-lang.org>
Wed, 20 Apr 2016 18:06:54 +0000 (11:06 -0700)
committerbors <bors@rust-lang.org>
Wed, 20 Apr 2016 18:06:54 +0000 (11:06 -0700)
Add `cargo doc --lib/--bin`

Allows documenting a specific target.

Closes #2557


Trivial merge